browsed image size sets to 0,0

Have just got CKfinder setup was looking OK then I started having this problem.

1) I click the insert image button on FCKeditor, the image properties popup opens.
2) I click browse and upload/locate an image, double clicking to return to image properties.
3) The preview loads, above 'Lorem ipsum', whilst its loading the width and height are blank.
...all OK so far then...
4) As soon as it has finished loading the images width should appear in the width/height boxes. Instead 0 appears in each and the preview image seems to vanish. It has just had its size set to 0,0. If i push the 'refresh size' button it is restored.

I have looked all through the forum and spent the last hour inserting images trying to find reliable behavior but I can't. 90% of the time the problem happens, occasionally it doesn't. Have tried clearing cache, restarting editor etc but it's not clear.

Would be deeply grateful if anyone can help.

Simon

PS. CKfinder php / FCK 2.5.1 / firefox
Hooray for FCK by the way - thank you